TOKYO (AFP) - Researchers in Japan said Thursday they had developed a flu vaccine that works against multiple viruses and could prevent a deadly pandemic of bird flu mutations. The research team has tested the vaccine on mice implanted with human genes, confirming that it works even if flu viruses mutate, according to Tetsuya Uchida, researcher at the National Institute of Infectious Diseases. Currently flu vaccines use a protein covering the surface of viruses but the protein frequently mutates to make the vaccines ineffective. The newly developed vaccine is based on common types of protein inside the bodies of flu viruses as they rarely change, Uchida told AFP. The viruses used are the Soviet-A and Hongkong-A along with the deadly H5N1 strain of bird flu. "We expect this will also be effective on new variations" of the much-feared H5N1 strain in addition to conventional flu viruses, he said. The World Health Organisation (WHO) warns that millions of people could die worldwide if the avian influenza virus mutates into a form easily transmissible among humans. Uchida said it would likely take several years to put the vaccine to practical use as the research team needs to confirm the vaccine's safety with further experiments on mice and possibly larger animals before tests on humans. The study is being jointly conducted by researchers from the national institute, Hokkaido University, Saitama Medical University and NOF Corp., a chemicals company based in Tokyo. Similar vaccination studies on attacking the inside of the virus body rather than its surface are also under way abroad, including at Oxford University in Britain, Uchida said. About 250 people have died of avian flu since 2003, according to the WHO. Indonesia is the country worst-hit by avian influenza with 115 deaths officially recorded since 2003. Five people have fallen victim in China this year. Human victims consist mostly of people in close contact with sick birds. There is no evidence so far that the deadly strain of bird flu has mutated into a form that could set off a pandemic.